A new class of polymers has come into existence in the last few years which are usually referred to as high performance thermoplastics. Generally, the concept of high performance always contemplates a polymer which has high heat resistance characteristics. Usually such a polymer will also possess excellent mechanical and electrical properties over a wide temperature range and is stable at elevated temperatures for extended periods of time. Generally, but not always, this class of polymers is soluble in a relatively small number of solvents and also has unusually excellent properties at very low temperatures.
As a general proposition, although not always, such polymers contain ring structures which are responsible for their high performance characteristics. Such rings comprise one or more of aromatic rings, the imide rings, the parabanic acid ring, and other heterocyclic rings usually containing carbon, nitrogen and oxygen therein. Although all such rings are relatively stable to high temperatures, the aromatic rings are relatively more so than heterocyclic rings.
In many instances, these ring-containing polymers also contain connecting linkages with varying degrees of stability. If the connecting linkage is considerably less resistant to adverse conditions than the ring components themselves, then the polymer will undergo degradation comprising either thermal oxidative chain scission or cross-linking reactions, or a combination of all at temperatures and under conditions far less severe than the ring itself is capable of withstanding.
In such event, the classic trite statement to the effect that the chain is no stronger than its weakest link is fully applicable. Therefore, it would be highly advantageous if techniques, compositions, and additives were developed which would enable one to overcome the effect of a particularly weak link in an otherwise very high performance polymer chain.
A very good example of such a weak link is the methylene group (--CH.sub.2 --) attached to at least one aromatic ring which is in a great number of polymers and is known to oxidize readily at relatively low temperatures. For the purpose of this application, a methylene linkage attached to one aromatic ring will be referred to as a methylene monoarylene group and if attached to two aromatic rings will be referred to herein as methylene diarylene. Such groups will be referred to generically herein as "arylmethylene."
In contrast, a linkage which appears in many high performance polymers is the diaryl ether linkage (aromatic ring-O-aromatic ring). It is not nearly as unstable as the arylmethylene group, but the polymer precursors in which it appears are considerably more expensive than the precursors in which the arylmethylene group appears.
Thus, it would be most desirable to utilize the relatively inexpensive arylmethylene-containing polymer precursors which are widely available with modest stabilization expenses, rather than to utilize the relatively expensive diaryl ether-containing precursor, even though the latter is known to result in a quite stable high performance polymer.